The emergence of spatially modulated electronic states and symmetry-breaking phenomena in unconventional superconductors has opened new perspectives for understanding the mechanism of high-temperature (high-T_c) superconductivity. Experimentally, the pair density wave (PDW) state, in which the superconducting order parameter exhibits a spatial modulation due to finite-momentum Cooper pairing, has been observed in unconventional superconductors and is hypothesized to play a fundamental role in high-T_c superconductivity. Using molecular beam epitaxy and scanning tunneling microscopy (STM), we observed a PDW with a period of about 3.6a_Fe (where a_Fe is the distance between neighboring Fe atoms) at domain walls in monolayer FeTe_0.5Se_0.5 films grown on SrTiO₃ (STO) [1]. By further performing STM measurements with sub-unit-cell resolution on monolayer FeTe_1-xSe_x/STO (x ≈ 1 and 0.7), we observed the intra-unit-cell superconductivity pair density modulations (PDM) characterized by the superconducting gap size and the coherence peak sharpness. The local maxima and minima in the superconducting gap modulation are centered at the locations of the chalcogen atoms, revealing the breaking of the glide-mirror symmetry of the chalcogen atoms [2]. Futhermore, by tuning the growth conditions, we detected a commensurate CDW order with a period of 4a_Fe in monolayer FeTe_0.3Se_0.7 film. Further analyses demonstrate that the observed CDW is a smectic order, which breaks both translation and C4 rotational symmetry. Moreover, the nematicity of the CDW order is closely related to the superconducting state [3]. These results provide a new route for studying the complex intertwined orders and their interplay in high-T_c superconductors.

[1] Yanzhao Liu#, Tianheng Wei#, Guanyang He, Yi Zhang, Ziqiang Wang and Jian Wang, "Pair density wave state in a monolayer high-T_c iron-based superconductor", Nature 618, 934–939 (2023).

[2] Tianheng Wei#, Yanzhao Liu#, Wei Ren#, Zhen Liang, Ziqiang Wang and Jian Wang. "Observation of Superconducting Pair Density Modulation within Lattice Unit Cell." Chinese Physics Letters 42, 027404 (2025) (Cover Page, Express Letter).

[3] Tianheng Wei#, Yanzhao Liu#, Wei Ren#, Ziqiang Wang and Jian Wang. "Intertwined charge and pair density orders in a monolayer high-T_c iron-based superconductor". arXiv:2305.17991.